Solve for $p$ : $28 = p + 13$
Subtract $13$ from both sides: $ \begin{eqnarray} \\ \hphantom{28 {- 13}} &\hphantom{=}& \hphantom{p} \\ 28 &=& p + 13 \\ \\ {-13} && {-13} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 28 {- 13} &=& p \\ \end{eqnarray} $ Simplify. $ p = 15$